Where's My Droid and Speed Night were cited as the most downloaded malicious apps

The number of mobile apps infected with malware in Google's Play store nearly quadrupled between 2011 and 2013, a security group has reported.

In 2011, there were approximately 11,000 apps in Google's mobile marketplace that contained malicious software capable of stealing people's data and committing fraud, according to the results of a study published Wednesday by RiskIQ, an online security services company. By 2013, more than 42,000 apps in Google's store contained spyware and information-stealing Trojan programs, researchers said.

Apps designed to personalize people's Android-based phones were most susceptible, as well as entertainment and gaming apps. Some of the most malicious apps in the Google Play store downloaded since 2011 were Wallpaper Dragon Ball, a wallpaper app, and the games Finger Hockey and Subway Surfers Free Tips.

Both Wallpaper Dragon Ball and Finger Hockey, RiskIQ said, have malware that steals confidential information such as device IDs from infected devices. Subway Surfers Free Tips, meanwhile, uses a Trojan called Air Push to bypass a device's security settings and subscribe infected phones to premium services, the company said.

RiskIQ performed its analysis using its own software that crawls app stores, websites and web ads. The technology, the company said, exposes malware that would otherwise not show itself to traditional web crawler software.

Android apps were only counted as being malicious if they behaved in specific ways as a result of malware. The behavior may include: collecting and sending GPS coordinates, contact lists and e-mail addresses to third parties; recording phone conversations and sending them to attackers; taking control of the infected phone; or downloading other malware onto the phone.

Apps in Apple's store were not analyzed.

The findings show that the rising prominence of mobile apps among consumers also makes them a juicy target for hackers. Reports of possible malware in clones of the popular Flappy Bird mobile game recently surfaced, even after it was removed from app stores.

"The explosive growth of mobile apps has attracted a criminal element looking for new ways to distribute malware that can be used to commit fraud, identity theft and steal confidential data," said RiskIQ CEO Elias Manousos, in announcing the findings.

Malicious apps are an effective way to infect users, he said, since they often exploit the trust people have in brands and companies they do business with.

But while the number of malicious Android apps is rising, the percentage of them removed by Google is on the decline, researchers said. In 2011 Google removed 60 percent of malicious apps, but in 2013 the company removed less than a quarter of them, the report said.

That's probably due to the rapid increase in malicious software. The overall number of malicious apps removed by Google still increased from roughly 7,000 in 2011 to nearly 10,000 in 2013.

Google said it would need more information about RiskIQ's analysis to comment on the findings.